A standard apparatus for hot-forming and press-quenching a semifinished workpiece made of hardenable steel has a die with a cavity and a punch with a complementary shaping part which are movable relative to one another, and a hold-down element. The apparatus further has a cutting device having an upper and a lower blade. Such an apparatus is used for shaping, cutting, and press-hardening a hardenable-steel workpiece that is loaded into the apparatus when heated above the AC3 point.
Growing demands on modern structures, particularly in automotive manufacturing, with respect to impact behavior and minimal fuel consumption are increasingly requiring the use of high-strength and ultrahigh-strength steels. Hot-forming and press-quenching, known from GB 1,490,535, are frequently used in this regard. This patent discloses a method for press-forming and hardening sheet steel having low thickness and good dimensional stability, in which a sheet made of boron alloy steel is heated to a temperature above the AC3 point, and within less than 5 seconds is pressed into the final shape between two indirectly cooled tools, resulting in a significant change in shape, and while still inside the press is subjected to rapid cooling in such a way that a martensitic and/or bainitic structure is obtained. Use of these measures results in a workpiece with high dimensional accuracy, good dimensional stability, and high strength values which is well suited for structural and safety-related parts in automotive manufacturing. Preshaped components as well as flat sheets may be hot-formed and press-quenched. For preshaped components, the shaping process may also be limited to shaping of a small percentage of the final geometry, or to sizing.
After the hot-forming and press-quenching, such high-strength and ultrahigh-strength steels have tensile strengths greater than 1000 MPa. The hardened components can then be cut only by using special processes. One common process in industrial mass production is laser cutting, which, however, is carried out on the hardened component as an additional production step following the hot-forming and press-quenching, and involves long cycling times and high capital costs. For this reason the process has a precise tolerance.
Another production process performed after hot-forming and press-quenching is so-called hard cutting, as disclosed in US 2007/0062360, for example. Hard cutting is performed on the hardened component, using special presses. This reference discloses a press and a method for hard cutting of a workpiece, in which the press has spacers located as close as possible to the cutting zone. At the moment of fracture of the workpiece, the spacers form a fixed stop which brings the motion of the press in the cutting direction to a standstill. This allows a zero gap to be provided between the upper blade and the lower blade. As a result, wear, noise generation, and cutting shock are minimized and a particularly fine cutting profile is achieved with minimal burr formation. However, the hard cutting process lengthens the production chain and requires additional complicated tools which are maintenance-intensive.
Therefore, it would be advantageous if a component required no trimming at all after hot-forming and press-quenching. US 2006/0137779 discloses a method for producing a metallic shaped component, in which a component blank is produced from a semifinished workpiece made of unhardened heat-treatable sheet steel, using a cold forming process such as deep drawing. The component blank is then cut on the edges to form a border shape approximating that of the component to be produced. The component blank trimmed in this manner is then heated and press quenched in a hot-forming tool. However, the heating, hot-forming, and press-quenching always cause a change in the component compared to the unheated state. Subsequent trimming can therefore be omitted only if the tolerance specification for the component is so broad that this change is still within the allowable tolerance. However, the allowable tolerance specifications are often narrower.
The same applies for so-called direct hot-forming, in which in a single shaping step a finished component is shaped and press quenched from a heated sheet. The sheet metal blank is first optimized in a prototype tool until no trimming is necessary after the shaping and press-quenching. However, direct hot-forming requires simple geometries and broad tolerance specifications. Trimming is unavoidable for more complex components with narrow tolerances.
Therefore, it would be practical to perform necessary trimming at a time when the component is no longer undergoing a change in shape and the hardening has not yet been completed. This point in time is located during the actual hot-forming, as soon as the contour is shaped and before the holding period. A generic hot-forming apparatus having cutting tools and a corresponding method are known from DE 10 2006 026 805. This reference discloses an apparatus for hot-forming and press-quenching of a semifinished workpiece made of hardenable steel, having a die with a front face and a punch with a protruding shaping part which are movable relative to one another, and a hold-down element, and the apparatus, which may be activated during and/or after the shaping process, including a cutting device having an upper and a lower blade. The trimming is advantageously carried out in the shaping apparatus, i.e. when the component is still warm. Thus, at this point the component does not have high strength, since it has not yet completely cooled and therefore is not fully hardened. In addition, separate tools or devices are not required; all that is needed is a single apparatus for hot-forming which also includes at least one cutting device. Furthermore, the hold-down element may be supported so that it is movable relative to the punch and/or the die. The hold-down element may also be connected to the punch, in particular in a spring-loaded manner. For discharging scrap pieces, the hold-down element advantageously has at least one oblique outer surface, whereby the scrap pieces separated from the component are discharged from the side of the apparatus. The step of trimming the component advantageously begins before its final shaping, particularly preferably with a trimming allowance with respect to the final shaping of the component. The final shaping is not performed until a final stroke of 1 mm, which causes the material of the semifinished workpiece to be indented slightly, i.e., drawn inward, in the region of the cut edge. In addition to trimming of the circumferential edge of the semifinished workpiece or the almost completely formed component, perforations or punches may be provided within the face of the semifinished workpiece or component during the cutting process. Alternatively, the punch may be moved in the direction of the die. A problem with the apparatus disclosed in DE 10 2006 026 805 is the fact that the trimming is performed just before the end of the shaping process. Tolerances can then be ensured only for simple cut edges. For complex components such as a B-column for a motor vehicle, the tolerances to be observed become problematic because the indentation of the material after cutting is not linear.